1. Field of the invention
This invention relates to a brushless motor drive circuit applying a predetermined magnitude of drive voltages sequentially to a plurality of stator coils with a predetermined phase angle, and more particularly to such a brushless motor drive circuit arranged so that the drive voltages are obtained from magnetic sensors delivering voltage signals in response to a magnetic flux density from a rotor formed by a permanent magnet.
2. Description of the prior art
Generally, a brushless motor comprises a plurality of stator coils, a rotor formed by a permanent magnet, and a plurality of Hall elements serving as magnetic sensors each producing a rotor position signal in response to a magnetic flux density from the rotor passing. The drive voltages applied to the stator coils respectively are sequentially switched in synchronism with rotation of the rotor based on the rotor position signals from the respective Hall elements. The drive voltages applied to the respective stator coils are obtained either from a usual power supply circuit or by amplifying the output voltages from the Hall elements. In the latter case, particularly, multiplication circuits are provided across output lines of the Hall elements respectively so that the magnitude of the drive voltages are controlled for the purpose of controlling the rotational speed of the motor by means of speed control signals. Each multiplication circuit is arranged to perform an analog multiplication so that the output voltage from the corresponding Hall element takes a value in accordance with the magnitude of the speed control signal. In the prior art, however, the cost of the drive circuit incorporating the multiplication circuits are disadvatageously increased since the multiplication circuits are complicate.